Energizing system for discharge tubes



Sept. 13,- 1938. E. G. DoRGELo 2,130,077

' ENERGIZING SYSTEM FOR DISCHARGE TUBES Filed Feb. 25, 1957 Q Tili l Hummm,

Patented Sept. 13, 1938 UNITED STATES 2,130,077 l ENEnGlzING SYSTEM son DISCHARGE TUB Eduard Gerardus Dorgelo, Eindhoven, Netherlands, assigner to N.

V. Philips Gloeilampenfabrieken, Eindhoven, Netherlands Application February 2'5, 1937, serial No. 127,756 In Germany February 28, 1936 5 Claims.

My invention relates to apparatus for recording or projecting picture or sound-picture films, and more particularly to a system for -periodically energizing the light sources of such apparatus.

As a light source in such apparatus it has been proposed to use artificially-cooled high-pressure metal-vapor discharge tubes, particularly those of the type described in the copending U. S. Patent application Ser. No. 46,952, flied Oct. 26, 1935 to Cornelius Bol et al., now Patent #2,094,694. Such tubes have many properties which make them particularly adapted for this purpose, Afor example when energized with an alternating current their light-emission curves have dark periods of definite duration, which may be used to render the jerlgv movement of the lm past the picture aperture invisible. and to eliminate the use of a shutter.

However, when the energizing current is of standard frequency, the length of the dark periods produced each time the alternating voltage curve passes through zero are too short for the above-mentioned purpose. More particularly, if a 50 cycle A. C. supply is used, the tube will be illuminated a hundred times per second, and thus each picture of a film travelling past the picture aperture at a speed of 25 pictures per second will be exposed four times. As a result the dark periods are very short, in fact so short that it will generally be necessary to use a shutter to obtain satisfactory projection.

Dark periods of suillcient length may be obtained if the tube is energized with 25 cycle current, particularly if a choke coil or a resistance is connected in series with the tube. However, this does not provide a reliable ignition of the tube in each cycle because generally the starting voltage of the tube must be rather high with respect to the maximum voltage of the A. C. supply to obtain dark periods of sufficient length. Although reliable ignition can be secured by applying to the tube, a higher A. C. voltage of the same frequency, this again reduces the length of the dark periods and satisfactory projection is not obtained, particularly without a'shutter. In addition, to obtain 25 cycle current from the standard supply mains it is necessary to provide a frequency transformer which complicates and increases the cost ofthe apparatus.

Although it has been proposed to use standard-frequency energizing current,v and to suppress one half of each cycle, in this case the dark periods become too long and the starting voltage 1 of the tube is substantially increased due, for instance, to reduction of the ionization in the dis- (c'l. 17e-124) charge chamber. As this reduces the reliability of the discharge process, it is necessary to use a higher A. C. voltage which generally'decreases the life of the tube. While the tube operating voltage could be limited to the desired value by the use of a; specially-dimensioned impedance in series with the tube, the energy supplied to the tube when initially applying voltage thereto (due to the size of the impedance required) would be tool small to heat the tube sufficiently fast. As a result the pressure of the filling of the tube cannot attain the desired high value and the proper illuminating intensity is not obtained.

The object of my invention is to overcome the above conflicting diiliculties, and to provide a. simple and inexpensive prjecting or recording film apparatus, which does not require a shutter.

In accordance with the invention, which is based on suppressing a portion of each cycle of an alternating supply current, I superpose upon the portion of the current which is not suppressed, a current component of another frequency; preferably of lower frequency than the supply current. Thereby, I regulate the length of the periods by altering frequency of the superposed current. l

More particularly, I use as tne light source, one or more cooled high-pressure metal-vapor discharge tubes having contracted discharge path, and energize same from an A, C. source of preferably standard frequency, through a circuit-arrangement consisting of two circuits connected in parallel with the source and including a common condenser; one circuit being a s eriesconnection of the condenser, a rectifier, and an impedance (preferably an ohmic resistance), and the second circuit being a series-connection of the condenser, an impedance, and the discharge tube or tubes.

The apparatus according to the invention provides proper starting and operating voltages for the tubes to ensure reliable ignition under all conditions as well as reliability of operation and long life. y l

In addition, by selecting proper values for the various elements of the circuit, I can obtain dark periods of any duration desired for this purpose. Furthermore, the length of the dark periods may be so selected that a shutter can be dispensed with without deleteriously affecting the quality of picture reproduction on the screen.

Further features and advantages of my invention will appear as the description progresses.

In order that my invention may be clearly un-4 derstood and readily carried into effect I shall describe same more fully with reference to the accompanying drawing, in which:

Figure 1 is a schematic circuit diagram illustrating the invention as applied to a projectingapparatus;

Fig. la is a schematic diagram of an energizing system according to the invention;

Fig. 2 is a sectional view of the discharge tube of Figure 1;

Figs. 3 and 4 are .voltage and current curves for use in explaining the invention.

Referring to Figure 1, an A. C. supply e, which may be the ordinary 60 cycle supply mains or a transformer, has a terminal I connected through a switch 30 to one electrode of a condenser C whose other electrode is connected through a rectifier G, for example a thermionic tube or a copper-oxide rectifier, to a resistance R, whose second terminal is connected to terminal 2 of the voltage supply e. In parallel with this series circuit and having portions in common therewith, is a second series circuit formed by condenser C, an inductance L, and 'a discharge tube E; the inductance and tube being connected in series across points II and I2. Resistance R may also be inserted between points 3 and 4, or between points 5 and 6 as shown in Figure 1a. A re sistance may be used instead of inductance L.

As the discharge tube E, I prefer to use cooled high-pressure metal-vapor discharge tubes such as described in the above-mentioned patent. Preferably the tube should be liquid-cooled and have a mercury-vapor filling of high-pressure, preferably more than 6 atmospheres, for example 150 atmospheres, and should comprise one or more incandescible electrodes protruding only slightly from a vaporizable metallic mass which surrounds said electrodes and which contains mercury or amalgam. With such discharge tubes an intrinsic brilliance of 20,000 International candles per sq. cm. and upwards, e. g. from 80,000 to 100,000 and more can readily be obtained, while the spectral composition of the light fully meets the requirements to be complied with for faultless projection. Such a tube is illustrated in Figure 2 in which the reference numeral 3l indicates a transparent envelope, e. g. of quartz, in which are disposed two mercury electrodes 32. 'I'he tube is surrounded by a cooling jacket 33 of transparent material through which cooling water is circulated as indicated by the arrows.

The circuit-arrangement shown in Figure 1 operates as follows; when terminal 2 is positive and terminal I is negative, condenser C becomes charged to a potential determined by resistance R, because rectifier G passes current only in this direction. When the polarity of terminals I and 2 is reversed, there will be applied across discharge tube E, a total voltage equal to the sum of the voltages of condenser C and of supply e. Thus, it is seen that the circuit-arrangement of Figure 1 permits the use -of discharge tubes having a starting voltagel and operating voltage which are higher than the maximum voltage of the A. C. supply, and that these tubes will be ignited and maintained in operation in a reliable manner.

The operation of the circuit-arrangement of Figure 1 will be more clearly understood by reierring to Fig. 3, in which curve 'I is the voltage curve of A. C. supply e. Upon closure of switch 30, the voltage of curve 'I attains a value 8 correand the latter passes current with the result that condenser C becomes charged. Whether or not condenser C will be charged to the highest attainable potential, i. e. to the maximum voltage I Il of supply e, depends on the value of resistance R which limits the condenser charging current. In the example under consideration, resistance R is so selected that the voltage to which condenser C is charged approximates the maximum voltage of source e. The voltage across condenser C is indicated by curve 9 from which it appears that condenser C retains its charge when curve 'I becomes negative because rectifier G does not pass current under these conditions.

When the voltage of supply e has attained its maximum positive value at I0, it decreases where by a potential difference, as diagrammatically indicated in Figure 3 by a hatched area, is set up across points II and I2 of Fig. 1. The seriesconnection of discharge tube E and choke L is connected between points II and I2, whereby when the voltage across points I I and I2 reaches the value of the ignition voltage of tube E, the tube will be illuminated. In Fig. 3, the starting voltage of the tube E is indicated by line I8 occurring at point I3. As indicated by current curve Il, the current through tube E then starts at a point I9 on line I8.

Condenser C now discharges through tube E, as indicated by the voltage-drop of portion I5 of curve 9, and at point I6 on curve 9, the potential difl'erence across points II and I2 of Fig. 1 become zero. However, there will still be a deflnite voltage applied across tube E because the current through choke L lags in known manner behind the voltage applied across points II and I2, and the self-induction voltage set up in this way across choke L will produce through tube E a current flow which continues for some time with decreasing intensity discharging the condenser till it becomes Arecharged 8 and stops at I'I.

Beyond point I6 the condenser voltage, as shown by curve 9, decreases until the supply voltage exceeds the condenser voltage by the amount o1' the ignition voltage of rectifier G, as indicated by line 8. At this point the charging of the condenser again starts and the process is repeated regularly.

As appears from Fig. 3, there will be produced regularly-occurring dark periods D whose duration may be determined by properly selecting the values of the elements of the system. The duration of the discharge of condenser C is determined by its capacity and by the inductance of choke L, and thus the duration of the dark periods D may be regulated by altering these quantities. If a resistance is substituted for choke L, the discharge of condenser C takes place so rapidly, that perods D become very long and the periods of light emission become very short.

Although the rate o1' charging condenser C and the voltage to which it is charged--thus the duration of dark period D-may be controlled to some extent by altering the value of resistance R, I prefer to regulate the duration of the dark periods by selecting proper values for condenser C and choke L. It will be clear without further explanation that the shape of current curve Il of tube E depends to an appreciable extent on the values of condenser C and choke L so that the shape of this curve and the duration of the dark periods can be controlled in a simple and reliable manner. Thevoltage of the supply e may, in this case, be selected so as to be as favorable as possible in respect to the operation of the tube. 75

From Fig. 4 it will more clearly appear that current curve I4 is mathematically composed of a feeding alternating current, to be referred to as steady current and a low-frequency current to be referred to as the compensating current, which latter currentL is also produced when switching an A. C. supply on an electric circuit comprising capacity and self-inductance (a tuned circuit), and has a frequency and magnitude substantially determined by the valuesl of condenser C and choke L.

In Fig. 4, in which the curves of Fig. 3 are denoted by the same reference numerals, reference numeral I9 indicates the moment of ignition of discharge tube E, at which moment current starts flowing through the tube and consequently the emission of light begins. The steady current, as indicated by curve 22, is out of phase relatively to the supply voltage curve 1, whereas curve 23, which indicates the compensating current, is of lower frequency than curve 22 and is determined by condenser C and choke L. At the'start of the discharge through tube E, i. e. at point I9, the sum of the currents of curves 22 and 23 is zero.

Thus it is seen that current curve I4 is the summation curve of the current curves 22 and 23. and that curve 23 generally has another, for example a lower, frequency than curve 22 which frequency is determined by the formula in which L and C are the inductance oi choke L and the capacity Aof condenser C respectively. The duration of every part or said compensating current 23, however, is determined by the duration of the tube supply current I4. 'I'his compensating current consists of a number, equal to the supply frequency, of separate equal parts.- It is evident that by altering the natural frequency wo, it is possible to alter the frequency of the compensating current", and thus the duration of the actual current passing through tube E; as well as the duration of the dark period.

As has been stated in connection with Fig. 3, the potential dill'erence between points II and I2 becomes zero at the point indicated by line I8-2IL Thus in Fig. 4, owing to the inductance of choke L, the actual current passing through tube E will substantially follow the course of a tangent 24 to a point I'I, and not to the nal point 2I of curve Il. Because of this, the portion of curve Il between points 2|! and I'I is not exactly equal to the sum of curves 22 and 23.

' It should be noted that the rectifier G does not function in the period between points I9 and 20 so that the parallel connection of the rectier has no appreciable inuence upon the discharge current. It is evident, however, that when the rectier subsequently starts functioning again. the sum of the steady curren and the compensating curren is equal to the total amount of the currents owing through the discharge tube and through the rectifler.

While I have described my invention with reference to specific examples and applications, I do not wish to be limited thereto but desire the appended claims to be construed as broadly as permissible in view of the prior art.

What I claim is:-

1. An energizing system for periodically energizing a high-pressure metal-vapor discharge tube having a contracted discharge path, comprising a source of alternating current, a circuit connected across said supply and having two branches, a condenser in said circuit and in series with both of said branches, a rectifier in one of said branches, an impedance in said circuit betweensaid rectifier and one end of said source, and a second impedance, said second impedance and discharge tubeA being connected in said second branch.

2. An energizing system for periodically energizing a high-pressure metal-vapor discharge tube having a contracted discharge path, comprising a source of alternating current, a circuit connected acrosssaid source and having two parallel branches, a condenser in said circuit and lin series with each of said branches, a 'rectifier in one of said branches, an ohmic resistance in said circuit between said rectifier and one side of said source, and an inductance, said inductance and discharge tube being connected in said second branch. i n

3. An energizing system for periodically energizing a high-pressure metal-vapor .discharge tube having a contracted discharge path, comprising a source of alternating current having a peak value less than the starting voltage of the discharge tube, a circuit connected across said source and having two parallel branches, a condenser connected in said circuit and in series with each of said branches, a rectier and an impedance in one of said branches, and a second impedance, said second impedance and discharge tube being connected in said second branch.

4. An energizing system for periodically energlzing a high-pressure metal-vapor discharge tube having a contracted discharge path, com.- prising a source of 60 cycle alternating current, a

circuit connected across said source and having two branches, a condenser connected in said circuit and in series with each of said branches, a rectier and an impedance in one of said branches, and a second impedance, said second impedance and discharge tube beingconnected in said second branch.

5. An energizing system for periodically energizing a high-pressure metal-vapor discharge tube having a contracted discharge path, comprising a source of alternating current, a circuit connected across said source and having two parallel branches, a condenser and an impedance in said circuit and in series with each of said branches, a rectifier in one of said branches, and

a second impedance, said second impedance and discharge tube being connected in said second branch.

EDUARD GERARDUS DORGELC. 

